Erasable digital video disk with reference clock track

ABSTRACT

An optical disk and compatible optical disk drive enabling erasable (rewritable) optical disks to have the same format and capacity as read-only or (recordable) write-once optical disks. A reference clock track and optional additional prerecorded phase synchronization patters are provided to enable writing of any random sector with frequency and phase matching of a random sector to the preceding and following sectors. The reference clock track and other phase synchronization patterns eliminate the need for preambles and extra space for speed variation. In a first embodiment, a disk has multiple layers, with at least one rewritable data layer and at least one reference layer. A spiral track on a surface of the reference layer has prerecorded patterns to be used for clocking. In a variation of first embodiment, the reference layer is also used for radial tracking control, eliminated the need for predefined tracks in the rewritable data layers. The reference layer is produced using the same technology as for read-only media, and is therefore very precise, low cost, and permanent. An additional laser system may be required to read the reference layer. The rewritable data layers may be unpatterned prior to writing. Alternatively, the rewritable data layers may include embossed sector or block headers to augment clock phase precision. In a second example embodiment, a single circular permanent (non-erasable) clock track is provided on a rewritable medium. The disk is then divided into radial zones, so that within each zone, the angular velocity of the disk is constant. A clock signal from the permanent clock track is then ratioed for each radial zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of Ser. No.09/605,949 (filed Jun. 28, 2000, now U.S. Pat. No. 6,310,844, which is adivisional application of Ser. No. 08/696,416 (filed Aug. 13, 1996) nowU.S. Pat. No. 6,118,753. Application Ser. No. 08/696,416 and 09/605,949are hereby incorporated herein by reference.

FIELD OF INVENTION

This invention relates generally to digital optical disks and morespecifically to erasable and rewritable digital optical disks.

BACKGROUND OF THE INVENTION

In general, there are three types of digital optical disks: read-only,recordable (also called Write-Once or Write-Once-Read-Many (WORM)) anderasable (also called rewritable). Examples of commercially availableread-only optical disk technologies are the Compact Disk (CD) fordigital audio and the Compact Disk-Read Only Memory (CD-ROM) forcomputer data. Compact Disk-Recordable (CD-R) drives and media are alsocommercially available. An example of an erasable (rewritable) opticaldisk technology is the Magneto-Optic disk, widely used for computer datastorage.

For any electromechanical disk technology, whether magnetic or optical,there are physical limitations on information density. Some of theselimitations are mechanical, for example, the tolerance on angularvelocity for a disk rotation control system, or the tolerance onpositioning a transducer relative to a data track. Other limitations areimposed by transducer or media physics, for example, diffraction effectsin optics. In some technologies, for example many computer magneticdisks, a disk remains in a single drive for writing and reading. If thesame drive writes and reads the disk, some repeatable mechanical effectsmay be ignored. For example, if the angular velocity of the disk is atthe high extreme of an allowable range, this is not a problem since adisk recorded at the high speed will be read at the same high speed, sothat data writing rates and data reading rates are identical. If a diskmust be interchanged (written in one drive and read in a differentdrive), then the various worse case limitations of both drives combine,and the tolerances for each drive must be narrowed in order to meet anoverall specification with interchange. For mass-produced read-onlymedia, for example CD-ROM's, the process for writing a metalized masterdisk for molding of copies is typically a very high precision (andexpensive) process. Very little of the overall allowable tolerances areused by the writing process, enabling most of the overall allowabletolerances to be used in the reading drives. Therefore, for example,portable read-only CD-ROM drives for notebook computers can havesubstantially broader tolerances (and substantially lower cost) than thedrives producing the master disks for molding. However, if each of thedrives involved in interchange can write data to a disk, then each ofthe drives must be limited to no more than half of any particularoverall tolerance specification. For example, if the angular velocity atany particular radial head position must be accurate to ±1.0%, theangular velocity for each drive involved in interchange must be accurateto ±0.5%.

Some data formats assume that an entire medium will be recorded at onetime. Other data formats assume that individual random sectors of datacan be erased and rewritten. Typically, format provisions for rewritingindividual sectors reduce the overall effective data capacity. CD,CD-ROM, and CD-R formats are designed for maximum data capacity with noprovision for rewriting individual sectors. CD-ROM's are organized intodata sectors and each data sector must be phase synchronized with thepreceding sector and with the following sector. As a result, compatibleCD-R disks must be written in sequence, starting from the first sectorand writing each sector in the physical order that they appear on thedisk. The standard CD format specifications do not support the abilityto write or overwrite individual sectors with random access or to appendto a partially recorded medium. Typically, recordable drives that canappend to a partially recorded medium, and drives that can erase andoverwrite previously recorded data, must provide data gaps foraccommodating angular speed variations between drives and must provideadditional clock synchronization patterns for accommodating clockdifferences between drives. For example, magnetic disks andmagneto-optic disks are typically formatted into sectors, with eachsector including a preamble for synchronizing a write clock, and witheach sector including extra space at the end to allow for variations inangular velocity, each of which reduces effective data capacity. CD,CD-ROM, and CD-R formats do not have sector preambles forsynchronization or extra space at the ends of sectors. In general,drives that can append or rewrite individual sectors with random accesshave a reduced effective disk capacity relative to drives, with the samebit density, that write an entire disk at one time.

In addition to clocking precision requirements and angular velocityprecision requirements, writing drives must meet precision requirementsfor radial position or track following. Writable and rewritable opticaldisk media often have a predefined track, typically a land and groovestructure. Other approaches to predefined tracks may be found in U.S.Pat. Nos. 5,213,859 and 5,204,852. Typically, for drives using groovesor similar approaches, the bandwidth and signal to noise ratio for trackcentering of the writing laser are not as good as that obtained by thehigh precision drives used for mastering read-only media. In addition,some servo approaches proposed for writable media may be incompatiblewith read-only formats.

Various digital optical disk standards are being planned in advance ofavailable technology. That is, various capacities and formats have beenproposed for future standardization, even though corresponding drivesand media may not yet be available or practical. An example is erasable(rewritable) Digital Versatile Disks (DVD) (previously called DigitalVideo Disks). The proposed standards for erasable DVD's assume that themodel established by CD's will continue. That is, the proposed standardsassume that at any particular bit density, read-only and recordable(write-once) media will have the highest possible data capacity anderasable (rewritable) media will have a reduced effective data capacity.The proposed standards assume that erasable (rewritable) systems musthave a lower capacity than read-only and recordable (write-once) systemsdue to the extra overhead for synchronization and gaps for drive speedvariation. The proposed standards assume that writable media must have aland and groove structure or other predefined track servo information.In general, format differences between proposed erasable disks andread-only disks are incompatible, so that drives must be designed toread two separate formats, or drives designed only for read-only andwrite-once disks will not be able to read erasable disks. The proposedevolution or “migration path” typically specifies that for each new stepin bit density, there will first be read-only products that extract themaximum possible capacity from the anticipated technology (shorterwavelength lasers and improved media characteristics), followed byrecordable (write-once) products having the same capacity as theread-only products, followed by erasable products with the same bitdensity as read-only and recordable products, but with a lower effectivecapacity.

There is a need for erasable (rewritable) optical disks having the sameformat and the same effective capacity as read-only and write-oncedisks.

SUMMARY OF THE INVENTION

A reference clock track and optional additional prerecorded phasesynchronization patters are provided to enable writing of any randomsector with frequency and phase matching to the preceding and followingsectors. The reference clock track and other phase synchronizationpatterns eliminate the need for preambles and extra space (gaps) forspeed variation.

In a first embodiment, a disk has multiple layers, with at least onedata layer and a reference surface. A spiral track on the referencesurface has prerecorded patterns to be used for clocking. In a variationof first embodiment, the reference surface is also used for radialtracking control, eliminated the need for predefined tracks in the datalayers. The reference surface is produced using the same technology asfor read-only media, and is therefore very precise, low cost, andpermanent. An additional laser system may be required to read thereference surface. The data layers may be unpatterned prior to writing.Alternatively, the data layers may include embossed sector or blockheaders to augment clock phase precision.

In a second example embodiment, a single circular permanent(non-erasable) clock track is provided on the erasable medium. The diskis then divided into radial zones, so that within each zone, the angularvelocity of the disk is constant. A clock signal from the permanentclock track is then ratioed for each radial zone. An additional lasersystem is required to read the clock track. However, in many drives, asecond laser will be required for backwards compatibility. The samesecond laser may be used for the clock track. The two embodiments arenot mutually exclusive and instead can be combined. As in the firstembodiment, the rewritable data area in the second embodiment mayinclude embossed sector or block headers to augment clock phaseprecision. Also, zones with constant angular velocity could beimplemented in the first embodiment.

Each of the example embodiments provides the following advantages:

1. The format and effective capacity of an erasable (rewritable) disk isidentical to the format and effective capacity of read-only andrecordable (write-once) disks. Either of the example embodiments mayalso be implemented with a write-once medium, enabling partial writingwith later appending.

2. Once written, the disk of either sample embodiment can be read in astandard read-only player. The reference surface (or clock track) andthe additional laser system (if required) are used only during writing.

3. The incremental cost of adding a reference surface or a clock trackis nominal.

The first embodiment (reference surface) provides the highest clockprecision of the two embodiments and also provides radial trackinginformation. The second embodiment (separate clock track) providessimplicity, lower cost, and improved random access times. The secondembodiment is simpler and may have lower cost because no additionallayers are added to the disk and because a second reading laser may bepresent anyway for other reasons. Zones with constant rotational speed,in either embodiment, may provide improved random access times.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram cross section of a digital opticaldisk and parts of an optical disk drive, in accordance with a firstexample embodiment invention.

FIG. 2 is a horizontal cross section of the optical disk of FIG. 1illustrating a spiral reference track on the reference layer.

FIG. 3A is a block diagram of a data track on the erasable layer of theoptical disk of FIG. 1, illustrating formatting into blocks.

FIG. 3B is a block diagram illustrating the logical format of the blocksof FIG. 3A.

FIG. 3C is the data track of FIG. 3A illustrating sectors within ablock.

FIG. 4 is a simplified top view of a digital optical disk in accordancewith a second example embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Proposed future digital optical disk configurations includesingle-sided-single-layer, single-sided-dual-layer,double-sided-single-layer and double-sided-dual-layer. For dual-layerdisks, an outer layer is sufficiently transparent to permit a readinglaser to focus through the outer layer onto an inner layer. Fordouble-sided disks, it is typically assumed that the disk will be“flipped” for reading either of the two sides with a single lasersystem, but duplicate laser systems may be provided for reading eitherside of the disk without flipping the disk. In general, proposed digitaloptical disks will have the same thickness (1.2 mm) and the samediameter (120 mm) as a compact disk.

FIG. 1 illustrates an example first embodiment of the invention in asingle-sided, single-data-layer configuration, but the reference layerof the first embodiment is generally applicable to any configuration. Inthe configuration illustrated in FIG. 1, an optical disk 100 comprisesmultiple layers. A top substrate layer 102 may be the same as the toplayer of a compact disk, typically polycarbonate. Layer 106 is anerasable (rewritable) data layer. Layer 106 may have protective coatings(104, 108) on each side and a partially reflective coating 110. Layer114 has a nonchangeable reference data surface 115 and also providesadditional support as a substrate layer. Layer 114 may be polycarbonatewith a reflective coating on surface 115 (for example, aluminum) withdata pits embossed in the reflective surface 115, just as data pits areembossed into a reflective surface for compact disks and CD-ROMs. Layer112 is a bonding material, bonding the reference layer 114 to therewritable data layer 106. A read/write laser may pass through substratelayer 102, protective coating 104, and erasable data layer 106 andreflect off of coating 110. For writing, a writing laser may physicallychange the transparency (for example, reversible color change of a dye)or reflectivity (for example, amorphous/crystalline phase change) ofsmall areas of the layer 106, so that a reading laser system senses achange in intensity of a laser passing through layer 106, reflectingfrom the partially reflective coating 108, and passing back throughlayer 106. In the embodiment illustrated in FIG. 1, the laser from aread/write laser system and the laser from a reference laser system passthrough a common objective lens, but each laser is focused onto adifferent layer of the disk 100. Various ways of achieving two differentfocal distances with a common objective lens will be discussed laterbelow.

The overall thickness of disk 100 may be about 1.2 mm (the same as acompact disk). Layer 114 may be about 0.6 mm thick (the same as theembossed polycarbonate substrate for a compact disk). The erasable datalayer 106 may be about 20-50 nm thick. The bonding layer 112 may beabout 30-50 μm thick. Embossed pits in the reference surface 115 may beabout 100-200 nm deep.

In FIG. 1, an optical head 116 has a single final objective lens 118shared by two laser systems, a reference laser system (120, 122) and aread/write laser system (124, 126). The reference laser system (120,122) is used only for reading the pitted reference surface 115.Reference number 120 depicts a laser source and reference number 122depicts a photodetector system. The read/write laser system (124, 126)is used for reading and writing on the erasable (rewritable) data layer106. Reference number 124 depicts a laser source and reference number126 depicts a photodetector system.

An electronic reference read system 128 reads patterns via the referencelaser system (120, 122) from the pitted surface 115 of the referencelayer 114, generates a synchronized clock signal 130, and optionallygenerates a servo signal 132. The electronic reference read system 128may optionally receive additional information via the read/write lasersystem (124, 126) (depicted by line 134) from the erasable (rewritable)data layer 106 to augment phase synchronization. An electronic datawrite system 136 controls writing on the erasable (rewritable) datalayer 106 by the read/write laser source 124. An electronic data readsystem 138 controls reading from the erasable (rewritable) data layer106.

The pitted reference surface 115 has a spiral track (FIG. 2, 200)containing bit patterns used to determine the frequency of clock signal130 for data that is written by the data write system 136 in the datalayer 106. In a variation, the patterns on the reference surface 115 mayalso be used for radial position control of the head 116 during writing.The reference surface 115 on reference layer 114 is mastered and molded(embossed) using a process that may be identical to the process formanufacturing read-only disks. Therefore, the reference surface 115 isvery precise but very low cost.

The erasable data layer 106 preferably has a format that is identical tothat of read-only disks. That is, once written, the disk 100 can be readin a standard read-only player. The reference surface 115 and thereference laser system (120, 122) are used only during the writingprocess.

In the first embodiment as illustrated in FIG. 1, the erasable datalayer 106 must be sufficiently transparent to enable focusing a laserthrough at least one data layer and onto the reference surface 115. Forexample, the erasable data layer 106 may be a partially transparentphase-change material. During writing, a laser beam from reference lasersource 120 is continuously focused through the final objective lens 118onto the embossed reference surface 115 and is used to read information(via reference photodetector system 122) from the reference surface 115.Enough light must penetrate through the data layer 106 and the partiallyreflective coating 110 so that the reference surface 115 can be readwhile the data layer 106 is being written. Examples of multiple layeroptical disks may be found, for example, in U.S. Pat. Nos. 5,202,875;5,255,262; 5,381,401 and 5,446,723.

At a minimum, the information on the reference surface 115 is used todetermine the frequency of the clock signal 130 used to write the dataonto the erasable data layer 106. Extra space at the end of each disksector in the erasable data layer 106 is not required because the datarate during writing is continuously adjusted to match the data rate ofthe pattern on the pitted reference surface 115. The information on thepitted reference surface 115 may also be used to determine the phase ofthe clock 130 used to write data. Since the data in each sector is phasesynchronized to the clock obtained from the reference surface 115, eachsector is phase synchronized to the preceding and following sectors,regardless of the order in which the sectors are written. Sectorpreambles in the erasable data layer 106 are not required. This enableswriting drives to be constructed with relatively inexpensive angularvelocity servo components while providing the capability to write a diskthat meets the specifications of read-only disks.

To maximize the bandwidth and signal to noise ratio of the referencesignal, the data pattern for the reference surface 115 is preferablyencoded at the maximum information density allowed by the technology,which is typically the same density as a read-only disk. An examplesuitable data pattern is 100100100100100 centered and repeating along aspiral track (FIG. 2, 200) on the reference surface 115. A highfrequency embedded servo pattern may also be included. Sector beginningand end markers may be included and sector numbers and track numbers maybe included. The key requirement for the reference surface pattern isthat it must provide a high bandwidth and low noise servo referencesignal that can be used by the clock synchronization circuit (referenceread system 128) and optionally by the track centering positioningsystem (servo 135). With a low noise and high bandwidth referencesignal, these systems can correct for errors induced by a lack ofprecision in the components in the writing drive.

The position of the reference surface 115 relative to the erasable datalayer 106 may shift slightly with time and temperature. In addition,because the reference laser 120 reading the reference surface 115 mustpass through the erasable data layer 106 twice, the phase of the clocksignal 130 obtained from the reference surface 115 may be sensitive todisk tilt and disk misalignment. Therefore, additional information maybe needed to control the phase of the write clock 130. Portions of thedata track on the erasable data layer 106 may be permanently embossed toaugment synchronization of the clock signal from the reference track200.

FIG. 3A illustrates a portion of a spiral data track 300 on the erasabledata layer 106. Some proposed optical disk format standards specify32-kbyte data blocks (302, 304). FIG. 3B illustrates one proposedlogical format of a data block (302, 304). Each block is logicallyformatted into 192 rows. Each row has a 32-bit synchronization mark 306,followed by 91 bytes of data 308, followed by another 32-bitsynchronization mark 310, followed by another 91 bytes of data 312,followed by an error correction code (ECC) field 314 for the row. Thelast rows provide ECC data (316, 318) for column error correction.

FIG. 3C illustrates the physical data track 300 of FIG. 3A, illustratingadditional detail within a block. Each block has 16 sectors (322), witheach sector having a header field (320). Proposed read-only formatstandards include sector headers but not the longer synchronizationpreambles required by proposed writable format standards. The 32-bitsync marks (306, 310) of FIG. 3B are illustrated in FIG. 3C asperiodically occurring every 91 bytes within each sector (322). Sectorheaders (320) and sync marks (306, 310) are fixed patterns and may bemade unchangeable (for example, by embossing these patterns on layer106). Physically, sync marks (306, 310) occur every 0.198 mm along adata track 300 and sector headers 320 occur every 5.15 mm along a datatrack 300. As an alternative, or as additional phase synchronizationaugmentation, approximately 2,976 bits at the end of each data block(part of the ECC data) can be replaced with an embossed preamble for thefollowing data block. This would allow recovery from a significant phaseerror, but would require an improved signal to noise ratio to compensatefor the reduction in error correction capability. Physical block lengthsare about 82.3 mm.

Providing embossed synchronization patterns on the erasable layer 106 inconjunction with the signal from the reference surface 115 would enablean even higher bandwidth reference for correcting the phase of the writeclock 130 (FIG. 1). Combining the high bandwidth frequency and phaseinformation from the reference surface 115 with a once-per-91-bytes oronce-per-sector or once-per-block phase reference from an erasable datalayer 106 enables a write clock 130 with a very precise phase, littledrift, and the ability to compensate for slowly varying misalignmentsbetween the data layer 106 and the reference surface 115.

Ideally, the spiral reference clock track 200 on the reference surface115 is also used to determine the position of the focused read/writelaser beam 124 relative to the center of the desired track 300 in theerasable data layer 106. This information can be used to control theactuator that moves the optical head 116 radially. Since the informationon the reference surface 115 is very precise, and can be made very highbandwidth (relative to alternative writable data surface servoinformation such as wobble grooves), the resulting closed loopperformance of the servo 135 can be very good. Therefore, data can bewritten in the data layer 106 with a precision that approaches theprecision obtained in the mastering and molding process used forread-only disks. As a result, a writing drive can obtain the requiredprecision during writing even though the writer is constructed withinexpensive components and subject to significant disturbances fromexternal shock and vibration.

Preferably, both the laser beam that reads a reference surface 115 andthe laser beam that writes in a data layer 106 are focused through thesame final objective lens 118. It is possible to use two separatelenses, but two lenses would require a precise connection and somethingto prevent their relative movements. Otherwise, keeping one lens ontrack would not keep the other lens on track. By using the same finalobjective lens 118, this difficulty is avoided.

Several techniques may be used to separately focus, through a singlefinal objective lens, the beam that is used for reading the referencelayer and the beam that is used for writing. One technique forseparately focusing two beams through one final objective lens is to usetwo different wavelengths for the two lasers, as depicted in FIG. 1. Forexample, the laser reading the reference layer can be red and the laserfor writing on the data layer can be blue or green. An advantage ofusing lasers with different wavelengths is that it is relatively easy toseparate the two returned beams.

In FIG. 1, plates 148 and 154 are wavelength sensitive mirrors, plate150 is a polarization sensitive mirror, and plate 152 is a quarter waveplate that rotates polarization by approximately 45° (wavelengthdependent). Light from laser 120 reflects from wavelength sensitiveplates 148 and 154 and light from laser 124 passes through thewavelength sensitive plates 148 and 154. Light from both lasers 120 and124 passes through polarization sensitive plate 150 with an initialpolarity, and each of the lasers passes through plate 152 twice for atotal rotation of approximately 90° (wavelength dependent) which causeseach of the lasers to then reflect off of the polarization sensitiveplate 150. As a result, light from laser 120 reflects from plate 148,passes through plate 150, passes through plate 152 twice, and then isreflected by plates 150 and 154. Light from laser 124 passes throughplates 148, 150, 152 (twice), is then reflected off of plate 150 andthen passes through plate 154.

Lens 118 may be wavelength sensitive to focus the two lasers at twodifferent layers on disk 100. Alternatively, or additionally, each lasercan have one or more separate lenses (140, 142) upstream from the finalobjective lens 118, that provide some focusing. Alternatively, theobjective lens 118 can have two slightly different curvatures. Forexample, the curvature in the center of the lens may focus on thereference surface 115 and the curvature at the periphery may focus onthe reflective coating 110 on the data layer 106. The laser for readingthe reference surface 115 would then pass through the center of thefinal objective lens 118 while the beam for writing to layer 106 passesthrough the periphery. This would have the advantage of using a slightlylower numerical aperture (NA) for reading the reference layer 114 andthe highest NA for writing. Using only the periphery of the lens hasbeen reported in the literature as “optical super resolution”. Itcreates a smaller central lobe of the focused beam at the expense oflarger amplitude side lobes and can be used to write smaller spots thancan be achieved with a gaussian shaped spot. For an example lens designproviding two focal points, see U.S. Pat. No. 5,446,565.

With a dual focus lens 118 and good control of the separation (bondinglayer 112) between the reference surface 115 and the erasable data layer106, one focus adjustment may be sufficient to focus on both layers. Ifthe separation cannot be well controlled, then some separate focusingmay be required. Again, upstream focus (for example, lenses 140 and 142)would easily provide the necessary adjustment.

In one present proposal for multiple layer optical disks, the datalayers have a thickness of 30-50 nm. The focal length of the objectivelens is typically about 600 μm, so that the necessary amount of upstreamfocus or the necessary change in the lens curvature is very small.

Even with both laser beams focused through the same final objective lens118, tilt of the surface of the disk 100 with respect to the lens 118must be limited. If the reference surface 115 is 50 μm farther from thelens 118 than the reflective coating 110 on the data layer 106, a tiltin the radial direction of 0.1 degrees would produce an off-trackmovement of about 10% of a track (proposed track pitch is 0.74 μm). Foracceptable error rates, if the drive uses the reference track 200 fortrack centering, 0.1 degrees is a reasonable design target limit fortilt. If the distance between layers is decreased, the acceptable tiltwould be increased accordingly.

FIG. 4 illustrates a digital optical disk in accordance with a secondexample embodiment of the invention. In FIG. 4, an erasable disk layer400 includes a permanent circular reference clock track 402. One lasersystem remains at the radial distance of the circular reference clocktrack 402 while a second laser system writes on a spiral data track 404(only partially illustrated).

CD's and proposed read only optical disks typically are written along asingle spiral track with a constant bit density along the spiral track.As the track spirals from a small inner radius to a large outer radius,the angular velocity of the disk must decrease to provide a constantlinear velocity (constant bit density for a constant frequency writingclock). For the CD-ROM standard, the constant linear velocity is 1.3meters/second. To accomplish this constant linear velocity, therotational speed of the disk changes from 500 rpm closest to the centerof the disk to 200 rpm at the outer edge of the disk. In general, forCD's, the random access time for reading a sector includes the timerequired to adjust the angular velocity of the disk. For generalbackground information, see Sherman, Chris, The CD ROM Handbook,Multiscience Press, Inc., 1988.

In the embodiment illustrated in FIG. 4, the frequency of the referenceclock derived from the reference clock track 402 will vary with theangular velocity of the disk. As the writing laser progresses toward theedge of the disk, the angular velocity of the disk will decrease, sothat the clock frequency read from the reference clock track 402 will begreater than the frequency required for writing. Therefore, in general,the frequency of the clock for writing must be ratioed from thefrequency of the signal from the reference clock track 402. In theembodiment illustrated in FIG. 4, the disk is divided into multipleradial zones (406-412). Within each radial zone, the writing clockfrequency and the angular velocity of the disk are kept constant. Withineach radial zone, the bit density varies slightly as the spiral trackradius changes. Since the angular velocity is constant within eachradial zone, random access times within a zone are improved because noangular velocity change is required. Since each zone requires a writeclock that is a fraction of the clock signal produced by the referenceclock track 402, embossed headers as discussed in conjunction with thefirst embodiment may be necessary to ensure precise phasesynchronization.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and various modifications as are suited to theparticular use contemplated. It is intended that the appended claims beconstrued to include other alternative embodiments of the inventionexcept insofar as limited by the prior art.

What is claimed is:
 1. An optical disk drive comprising: a first laser system reading a reference clock track on an optical disk; a second laser system reading and writing data on the optical disk; a clock circuit receiving a signal from the first laser system and using the signal from the first laser system to generate a clock signal to the second laser system for writing on the optical disk.
 2. The optical disk drive of claim 1, further comprising: the second laser system reading permanent sector headers on the optical disk; and the clock circuit receiving a signal from the second laser system generated from the permanent sector headers and using the signal from the second laser system to refine the phase of the clock signal.
 3. The optical disk drive of claim 1, further comprising: the second laser system reading permanent synchronization markers on the optical disk; and the clock circuit receiving a signal from the second laser system generated from the permanent synchronization markers and using the signal from the second laser system to refine the phase of the clock signal.
 4. The optical disk drive of claim 1, further comprising: the second laser system reading permanent block headers on the optical disk; and the clock circuit receiving a signal form the second laser system generated from the permanent block headers and using the signal from the second laser system to refine the phase of the clock signal.
 5. The optical disk drive of claim 1, further comprising: the second laser system reading permanent sector preambles on the optical disk, the sector preambles positioned in an error correction code area of a previous sector; and the clock circuit receiving a signal from the second laser system generated from the permanent sector preambles and using the signal from the second laser system to refine the phase of the clock signal. 